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Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
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Debt and Equity Securities, FV-NI [Line Items] |  |
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | $ 13,982 | $ 166,470
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (32) | (637)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 0 | 235,380
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | (2,950)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 13,982 | 401,850
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (32) | (3,587)
US Government Agency Debt Securities Commercial [Domain] |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value |  | 54,501
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | (201)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value |  | 16,365
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | (149)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total |  | 70,866
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss |  | (350)
Residential Mortgage Backed Securities [Member] |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 10,777 | 111,969
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (23) | (436)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 0 | 219,015
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | (2,801)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 10,777 | 330,984
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (23) | $ (3,237)
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member] |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 998 |
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (2) |
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 0 |
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 |
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 998 |
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ (2) |

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